The importance of sagittal plane analysis and global sagittal balance and the parameters associated therewith has been widely studied of late, such as in the articles entitled “Adult Spinal Deformity: Sagittal Imbalance” by J. M. Cavanilles-Walker et al, published in International Journal of Orthopedics, Vol. 1(3) pages 64-72 (2014), and “Correlation of Pelvic and Spinal Parameters in Adult Deformity Patients with Neutral Sagittal Imbalance” by D. Deinlein et al, published in Spine Deformity, vol. 1, pages 458-463 (2013), and “Use of Surgimap Spine In Sagittal Plane Analysis, Osteotomy Planning and Correction Calculation” by M. Akbar et al, published in Neurosurg. Clin. N. Am., Vol. 24, pages 163-172 (2013), and “Spino-Pelvic Parameters after Surgery can be Predicted” by V. Lafage et al, published in SPINE, Vol. 36, No. 13, pp. 1037-1045 (2011).
One of the primary objects in correction of spinal deformities is to enable the patient to maintain a natural balance without incurring any stress or pain to maintain that balance. In a normal person, the balance is determined by the various angles of the curvatures of the spine, and specific distances between certain spinal anatomies. These angles and distances, and especially the lumbar lordosis and the thoracic kyphosis in the Sagittal plane and certain pelvic parameters, and the Cobb angle in the Coronal plane and the apical vertebral translation have certain accepted “normal” values, including standard deviation margins, such that when a person with a normally curved spine stands straight, in a posture in which he has no or minimized back pain, these angles and linear distances are within the limits defined as normal As stated in the above referenced Cavanilles article, and others, a main purpose of the pelvic, lordotic and kyphotic spine segments is to balance the head over the pelvis in an energy-efficient position, allowing the C7 plumb line, a vertical line drawn from the center of the C7 vertebral body, to pass as close as possible to the posterior-superior corner of S1. This line is known as the sagittal vertical axis (SVA), and although it is only one alternative parameter used to determine correct spinal posture, it is currently one of the most commonly used radiographic parameters in the evaluation of sagittal plane deformities. Another such parameter in common use includes the T1 Spino-Pelvic Inclination (T1SP1). Additionally, a number of coronal measurements are also used.
Reference is now made to FIG. 1, as taught by J. Dubousset in the article “Three-dimensional analysis of the scoliotic deformity” published in the book “The Pediatric Spine: Principles and Practice” Raven Press, NY, pp. 479-496 (1994). FIG. 1 illustrates schematically what is known as the “Cone of Economy”, which is the spatial volume in which the subject feels that his/her body remains in an ergonomically favorable position of balance, with the optimum position being, of course, in the center of the cone. Deviations outside of this cone require external support for the subject to feel in balance. Any sufficiently serious spinal deformity can cause the subject's posture to fall outside of this cone, and there are a number of spinal parameters used to characterize when this situation occurs, as outlined hereinbelow.
In a subject having a healthy spinal deportment, the SVA should fall no further than a few millimeters from the posterior-superior corner of S1. If the subject has spinal deformities which cause a larger divergence of the SVA, the person may have the feeling that he is not in balance, and the brain constantly directs the person to strain his posture in order to attempt to correct his balance, this generally resulting in back pain, and is not always effective in restoring balance or stability of the patient. In order to maintain some sort of sagittal balance, the patient may use different compensatory mechanisms, such as pelvic retroversion, hip extension or knee flexion. The main objective of these mechanisms is to allow the patient to keep an erect position in an energy-efficient way. Maintaining these positions against the clinical status of the patient's spine generates fatigue and muscular or skeletal back pain, which may or may not require surgical correction, but once the spinal deformity is sufficiently large to surpass these compensatory mechanisms, surgical intervention is generally required.
Such surgical correction is designed to achieve restoration of sagittal plane alignment, and can include both fusion and osteotomy procedures, and soft tissue (ligament) release. Spinal osteotomies are often used as part of a spinal reconstructive procedure to achieve spinal balance, stability and correct spinal alignment. A wedge of bone is resected from part of one or more vertebral segments to realign the spine to a better curvature to provide better sagittal balance. Several different osteotomy procedures have been developed, including the Smith-Peterson osteotomy, pedicle subtraction osteotomy, and vertebral column resection, some of which have been in use for many decades. However, if the correction is not done accurately, the person may still have a feeling of imbalance and/or pain, and may continue to try and “straighten” his back, or correct his posture, without generally succeeding in overcoming his problem. As will be discussed hereinbelow, this situation may result either from the non-iterative nature of the programs used to plan spinal curvature correction, or from the often intuitive nature of the decisions made by the surgeon. Examples of these situations can be found in the article entitled “Long-term investigation of nonsurgical treatment for thoracolumbar and lumbar burst fractures: an outcome analysis in sight of spinopelvic balance” by H. Koller et al., published in Eur. Spine J. Vol. 17(8), pp. 1073-1095 (August 2008).
A number of software programs are available commercially, designed for assisting the doctor in planning the optimum surgical procedures in order to correct such spinal deformities. Most of these programs operate on 2-dimensional X-ray images, though some offer planning routines based on 3-D CT or MRI images data sets.
A large number of parameters for defining spinal geometry have been proposed. One currently available software suite, has over 20 parameters which the doctor can manipulate in planning a procedure to restore correct posture to the patient's spine. The software has the ability to calculate “over twenty measurements in a few simple clicks”, in order to plan correct sagittal alignment. The doctor uses the 2-D sagittal image of the spine and makes virtual adjustments of the vertebral positions on the image, generating greater or lesser curvatures in the image of spine, in order to obtain the estimated optimum curvature.
However, because of the complexity of the planning procedure, in practice, what is often done is that the surgeon makes the sectioning according to his best judgement and intuition when looking at the images. This arises because of the complexity and multiplicity of parameters which the doctor has to manipulate in order to select the correct resection procedure, as illustrated above. Additionally, the planning is performed on a preliminarily obtained static image (or set of images in the case of a 3-D imaging process), taken of the patient standing, but corrected for the different form of the spine between the supine and standing positions of the patient. In the case of an image obtained from a CT data set of the patient in a prone position, similar to that during surgery, such a correction is not necessary.
Some prior art software programs for predicting graphically a plan for corrective surgical intervention, suggest 2-dimensional correction and therefore don't demonstrate how the coronal correction, for instance, will affect the axial and sagittal planes and vice versa. Furthermore, the surgeon often plans corrections to the patient's spinal curvature on a local basis, regarding just a pair or few vertebrae for each correction step, and generally based on the surgeon's experience, knowledge and intuitiveness, rather than on measured parameters. There is therefore a need to augment this largely qualitative correction approach using quantitative data. Limitations of presently used methods may play a part in the recently estimated statistic that up to one third of kyphosis surgeries in adults are revisions of previously attempted corrections, and that up to 7% of scoliotic corrections in pediatric cases require revisions.
One of the problems of possible software programs for planning spinal correction surgery may be that they could be considered as operating in a manner similar to graphic engineering design programs. The patient's spine could be adjusted in the same way as an engineer may, for instance, design a product using a CAD program. In a graphical plan, anything is allowable to achieve the desired shape. However the patient's spinal structure has preexisting limitations by the clinical situation of each vertebra and vertebra. Therefore, although correction of a defective spinal curvature by osteotomy and fusion procedures may appear possible on a virtual planning program, the clinical limitations of the patient's back may not enable this procedure to be successfully accomplished. And even if such a correction is achieved, it may be at the expense of performing surgical procedures on more vertebra than may be required by a surgically more economic overall method, such as that to be described in the present disclosure.
There therefore exists a need for a system and method which can generate a surgical plan for spinal curvature correction, preferably taking into account the individual capabilities of the patient's motion range, and which provides a more accurate clinical correction plan for the patient's deformities.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.